In many radio frequency (RF) applications, the phase noise of a local oscillator (LO) clock source is a key concern in designing the system. In broadband applications such as telecommunication, phase noise is often expressed in frequency domain in units of decibels relative to the carrier per Hertz (dBc/Hz), or equivalently in time domain using terms such as integrated root mean square (RMS) jitter or total jitter.
Voltage controlled oscillators (VCOs) are electronic oscillators used in a number of electronic circuits. A VCO has a voltage input that controls the oscillation frequency. An inductor-capacitor (LC) VCO is a VCO that includes a frequency-selective resonance tank including an inductor and a capacitor. In designing a low phase noise VCO, the size of a gain stage of the VCO (also known as driver) can be optimized in order to improve driver signal-to-noise ratio (SNR). In many cases, driver noise due to active or resistive components in gain stage dominates the phase noise. Some ways for attempting to lower phase noise are known to those skilled in the art.
The phase noise of a single VCO can be improved, meaning reduced, up to a certain point by improving the quality factor (Q) of the LC resonance tank, referred to subsequently herein as an LC tank. The Q factor can be improved but at some point the inductance and the Q factor become too challenging to model and predict, and other implementation limitations arise.
A greater reduction in phase noise may be achieved by implementing two or more resonance LC tanks in parallel. This approach is known as multicore VCO or array VCO. A multicore VCO is a VCO comprising a plurality of connected inductor capacitor (LC) VCOs, or “cores”, and an averaging scheme for improving the collective phase noise of the VCO. The overall oscillator clock signal comprises an average of oscillation signals of the plurality of VCO cores. The multiple cores maybe arranged physically in an array, for example having one or more rows and columns.
The VCO cores, and thus their LC tanks, are connected in parallel. The oscillation phase noise improvement with a multicore VCO compared to a single core VCO is ideally 10*log10(N) decibels (dB), where N represents the number of cores coupled in-phase. For example, when N=2, a 3 dB improvement in phase noise may be achieved. The oscillation power of the multicore VCO increases by a factor of N whereas the random noise power of the VCO only increases by the square root of N.
FIG. 1 shows a multicore VCO 100 comprising multiple VCO cores 101, 102, 103, 104. The output of each core is connected to a coupling and averaging circuit 105. An example VCO core 101 comprises a driver 156, an inductor 150, a fixed capacitor 154, and a voltage-tunable variable capacitor, or varactor 152, the capacitance of which is determined by an input control voltage. The driver 156 has a transconductance gain ‘gm’ and provides sufficient gain to overcome the losses in the non-ideal inductance and capacitance elements and lossy interconnections to ensure that the criteria for generating oscillations are met. The inductor 150 and fixed capacitor 154 approximately set the natural resonant frequency of the VCO core. By tuning the control voltage of the varactor 152, the frequency of the VCO core can be fine-tuned. For example, a phase-locked loop (PLL) circuit can be constructed to lock the VCO's output signal frequency and phase to the frequency and phase of a system level input reference clock signal.
In the multicore VCO 100 in FIG. 1, the outputs of the VCO cores 101, 102, 103, 104 are connected to coupling and averaging circuit 105, which averages the individual outputs of the first to Nth VCO cores 101, 102, 103, 104. The signal output 140 of the coupling and averaging circuit 105 can be used as the overall oscillator output signal. Because the noise of each array element (VCO core) is uncorrelated from the others but the signals of the VCO elements in the array can be synchronized and correlated by means of coupling, the SNR of the average output signal of the array of VCO elements can be higher than the SNR of an individual VCO element's signal, hence the phase noise can be improved in the VCO array.
Conventionally, the desired characteristics (e.g. phase noise, electromagnetic interference (EMI), and/or power consumption characteristics) of a multicore VCO are selected at design time, and the VCO is designed, laid out and manufactured accordingly with no ability to vary the characteristics at a later time, other than the adjustment of the respective varactor 152 and/or capacitor 154. A considerable amount of time and effort for electromagnetic (EM) modeling and simulation may be required at the design stage. Characteristics include, for example, phase noise, EMI (e.g. near-field and/or far-field), power consumption, quality factor (Q), and frequency tuning range. One issue with a multicore VCO is that it can produce more EMI compared to a single core VCO as a result of there being multiple inductors, namely one or more inductors in each VCO core. Accordingly, in this regard there may be a tradeoff between phase noise and EMI of a multicore VCO.
Unfortunately, not all characteristics can be optimized in one design particularly as there are performance tradeoffs between characteristics. Furthermore, once a multicore VCO has been designed and manufactured, its performance and behavior may differ from those of the simulated design, for example due to manufacturing variations or inaccuracies introduced in modeling and simulations. However, existing multicore VCOs provide little or no flexibility for adjusting these tradeoffs depending on the specific circuit application in which the VCO is to be used or on the performance of the manufactured device.